1. Field of the Invention
The present invention relates generally to an optical disk device, and more particularly to reproduction a wobble signal in a recordable or rewritable optical disk device used with a CD-R (compact disk recordable) or a CD-RW (compact disk rewritable).
2. Description of the Related Art
Conventionally, in a recordable or rewritable optical disk such as a CD-R, CD-RW, DVD-R, DVD-RW, or DVD-RAM (digital versatile disk random-access memory), guide tracks are “wobbled” so as to record ATIP (absolute time in pregroove) or ADIP (address in pregroove) on the pregroove. The ATIP or the ADIP (hereinafter referred simply to as “address information”) is used to acquire the current position of the optical disk, and according to this address information, a control of the recording or reproduction is performed.
The address information can be reproduced by taking out wobble components contained in an electric signal obtained by receiving the light reflected from the optical disk. For instance, assuming that a photodetector comprised of four quadrant photodiodes (having light receiving surfaces A, B, C and D) is used with two light receiving surfaces combination A and D and two light receiving surfaces combination B and C being divided in a radial direction of the optical disk, output signals obtained when the reflection light EFM-modulated (eight to fourteen modulation) depending on the presence of pits are received on the two light receiving surfaces combinations are in phase with each other. On the other hand, wobble signals at the two light receiving surfaces combinations are 180° out of phase with each other. Accordingly, by differentiating the output signals to remove EFM-modulated components, a wobble signal can be extracted.
FIG. 7 shows a circuit diagram of a wobble signal reproducing system used in a conventional optical disk device. Two photodetectors A and B divided in the radial direction of an optical disk receive the light beams reflected from the optical disk and output electric signals corresponding to intensities of the reflected light beams (in the case of quadrant photodetectors, it may be considered that two light receiving surfaces together form a single photodetector A, and the remaining two light receiving surfaces together form the other single photodetector B). The output signal from the photodetector B is supplied to a sample-hold circuit (S/H) 50, and the output signal from the photodetector A is supplied to a sample-hold circuit (S/H) 52. The sample-hold circuits 50, 52 are supplied with a sampling pulse, which is synchronized with the timing of light beam power at a time of reproduction. Based on the sampling pulse, the sample-hold circuits 50, 52 perform sampling of the output signals in a period of the light beam of reproduction power.
The sampling in the period of the light beam of reproduction power is achieved at a time of recording where a light beam of recording power and a light beam of reproduction power are alternately irradiated on to the optical disk. At a time of reproduction for reading out information recorded on the optical disk, the light beam is always irradiated with reproduction power, so that the sample-hold circuits 50, 52 can be passed over.
The signals that have been sampled and held by the sample-hold circuits 50, 52 are then amplified by amplifiers 54, 56, respectively, at a predetermined amplification factor and subsequently supplied to a differentiator 58 that determines a difference between the two signals. The calculation of the difference by the differentiator 58 is achieved so as to remove EFM-modulated components. An output signal from the differentiator 58 is supplied to a band-pass filter (BPF) 60. The BPF 60 extracts from the input signal only components of frequencies near a predetermined wobble frequency (22 kHz, for example) and outputs the extracted components as a wobble signal.
FIG. 8 shows a timing chart illustrative of the manner in which the wobble signal is reproduced in the conventional optical disk device. At a time of recording, the light beam power is increased to melt part of a pigment layer on the surface of optical disk to thereby form a pit. In a recording operation, as indicated by (a) in FIG. 8, the reproduction power and the recording power appear in an alternate fashion. In general, since the length of a pit is in the range of 3T to 11T where T is a reference period in the “track” direction, the light beam power varies in a range of 3T to 11T. Conventionally, as indicated by (b) in FIG. 8, a wobble signal is reproduced by sampling and holding the output signal in a period of the light beam of reproduction power.
By thus reproducing the wobble signal within a period of the light beam of reproduction power, address information can be obtained at a time of recording. However, the conventional method is difficult to meet the recent demand for high-speed recording.
In a high-speed recording process, the reference period T becomes smaller as the rotational speed of the optical disk increases. Especially when the period of signals is set to be small such as 3T or 4T, it is difficult to perform sampling of the output signals from photodetectors. In general, the provability for the signals with short periods such as 3T to occur is relatively large with respect to the overall signals. Thus, a sampling failure in each of the short signal periods lowers the signal-to-noise (S/N) ratio of the wobbling signal. When the address information is demodulated based on a wobble signal with low S/N ratio, an error of demodulation may occur.
It is theoretically possible to provide a sample-hold circuit, which is capable of performing high-speed sampling required for high-speed recording. However, this measure in practice will make the device complicated in construction and costly to manufacture.